1. Field of the Invention
The present invention relates generally to solar cells and, more particularly, to a solar cell which enables prevention of an accident caused by a reverse bias voltage generated, for example, when a solar cell module is shaded.
2. Description of the Prior Art
FIG. 12 is a top plan view showing a typical example of a solar cell of the prior art; FIG. 13 is a cross-sectional view taken along the line 13--13' of FIG. 12; and FIG. 14 is a bottom view of the solar cell of FIG. 12. As can be seen in FIGS. 12, 13, and 14, a light receiving surface of the solar cell is coated with a transparent anti-reflection film 8. Comb tooth or grid-like from electrodes 7 are provided beneath the anti-reflection film 8. One end of each of the from electrodes 7 is connected to an electrode connecting portion 5 which is a so-called bus bar or contact electrode. Further, the solar cell includes a P type region 1 occupying a large part of a silicon substrate, an N type region 2 formed on an upper side surface of the P type region 1, and a P.sup.+ region 3 for a BSF (back surface field) effect, foraged on a lower side surface of the P type region 1. A back electrode 6 is provided to cover approximately the entire lower side surface of the P.sup.+ region 3.
The solar cell of FIG. 13 can be manufactured, for example, by the following process:
First, a P type silicon substrate 1 shown in FIG. 15A is prepared.
With reference to FIG. 15B, a P.sup.+ region 3 is formed by diffusion into the overall surface area of the P type silicon substrate 1 of FIG. 15A.
Referring to FIG. 15C, the P.sup.+ region 3 on the upper surface of the silicon substrate 1 is then removed by etching.
After that, an N type region 2 is formed on the upper surface of the P type silicon substrate 1 as shown in FIG. 15D.
With reference to FIG. 15E, comb tooth-like front electrodes 7 (not shown) and an electrode connecting portion 5 are formed on the N type region 2. At least the N type region 2 is coated with an anti-reflection film 8. A back electrode 6 is provided on the lower surface of the P.sup.+ region 3. With the resultant structure cut along the broken lines indicated on opposite sides of FIG. 15E, the solar cell shown in FIG. 13 is obtained.
It is rare that the solar cell shown in FIG. 12 is used alone. Normally, in order to obtain a desired output voltage and current, a solar cell module M in which a plurality of solar cells are connected in series and in parallel is formed, as shown in FIG. 16A.
When the solar cell module M is in practical use, a part of the module M is sometimes shaded. In the case with a solar cell module M for use in space, it is possible that the shadow of a part of a main body of a satellite or the shadow of a structure such as an antenna is cast on the solar cell module M during, for example, attitude control of the satellite. Further, in the case with a solar cell module M for terrestrial use, it sometimes happens that the shadows of trees or buildings are cast on the solar cell module M and that droppings of birds obscure part(s) of the solar cell module M.
Referring to FIGS. 16A and 16B, solar cell module M in which some of the solar cells are shaded is illustrated. That is, a shadow is cast on a submodule 10 including a plurality of solar cells arranged in parallel. With reference to FIG. 16A, a voltage V.sub.11 generated from another submodule 11 which unshaded is applied as a reverse bias voltage to the shaded submodule 10 in a shunt mode in which the opposite ends of the solar cell module M are almost short-circuited. A relation V.sub.10 =-V.sub.11 is satisfied where the voltage of the submodule 10 is V.sub.10. When the solar cell module M is connected with an external power supply V.sub.B, a relation V.sub.10 =V.sub.B -V.sub.11 is satisfied as shown in FIG. 16B.
That is to say, when a positive voltage is applied to the front electrodes of the shaded submodule 10 of the solar cell module M and the reverse bias voltage of the applied positive voltage exceeds the reverse breakdown voltage of the solar cell, the solar cells in the submodule 10 incur short-circuit damage, thereby deteriorating the output characteristics of the entire solar battery module M.
In order to prevent such an accident caused by the reverse bias voltage in the solar battery module M, a bypass diode is typically connected for each solar cell or each submodule including a certain number of solar cells. In addition, so-called diode integrated solar cells are employed in which the bypass diodes are integrated with the solar cells themselves.
The method of connecting the bypass diodes has problems such that the manufacturing cost of the solar cell module increases in proportion to the number of bypass diodes to be used and the light receiving area on the solar cell decreases in proportion to the area necessary for connection of the bypass diodes.
In addition, in the diode integrated solar cell, since both the diodes and the solar cells must be integrated in the same silicon substrate, its manufacturing method becomes complicated. That is, the manufacturing cost of the diode integrated solar cell becomes higher as compared to that of a normal solar cell.